1. Technical Field
The present invention relates to a group III nitride semiconductor light-emitting device that has a light-emitting diode (LED) structure and a method of manufacturing the same, and a lamp.
Priority is claimed on Japanese Patent Application No. 2009-054204 filed on Mar. 6, 2009 and Japanese Patent Application No. 2010-46812 filed on Mar. 3, 2010, the content of which is incorporated herein by reference.
2. Background Art
In recent years, a group III nitride semiconductor has been attracting attention as a semiconductor material used for a light-emitting device which emits light of a short wavelength. The group III nitride semiconductor is represented by a general formula AlxGayInzN (0≦x≦1, 0≦y≦1, 0≦z≦1 and x+y+z=1), and is formed on a substrate made of sapphire single-crystal, various kinds of oxides or a group III-V compound, through a metal organic chemical vapor deposition (MOCVD) method, a molecular beam epitaxy (MBE) method, or the like.
In a general light-emitting device using a group III nitride semiconductor, an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer formed of the group III nitride semiconductor are sequentially layered on a sapphire single-crystal substrate. Since the sapphire substrate is an insulator, the device structure generally has a structure in which a positive electrode formed on the p-type semiconductor layer and a negative electrode formed on the n-type semiconductor layer are present on the same plane. Such a group III nitride semiconductor light-emitting device has two types: a face up type in which a light-transmitting electrode is used as a positive electrode to extract light from the p-type semiconductor side; and a flip chip type in which a highly reflective film of Ag or the like is used as a positive electrode to extract light from the sapphire substrate side.
External quantum efficiency is used as an output indicator of such a light-emitting device. When the external quantum efficiency is high, it is possible to say that the light-emitting device has a high output. The external quantum efficiency is represented as the product of internal quantum efficiency and light extraction efficiency.
Further, the internal quantum efficiency refers to the proportion of energy converted into light in the light-emitting layer from energy of electric current applied to the device. The light extraction efficiency refers to the proportion of light that can be extracted to the outside of the light-emitting device from light generated in the light-emitting layer.
Accordingly, in order to improve the external quantum efficiency, it is necessary to improve the light extraction efficiency in addition to light emission efficiency in the light-emitting layer.
There are mainly two ways to improve the light extraction efficiency. One is a method of reducing light absorbed by an electrode or the like formed on a light extraction surface. The other one is a method of reducing light confinement within the light-emitting device occurring due to a difference in refractive index between the light-emitting device and an outside medium thereof.
In this regard, as a characteristic of a gallium nitride-based compound semiconductor device having the above-mentioned composition, for example, there is less diffusion of electric current in a transverse direction. For this reason, electric current is applied to only a semiconductor portion directly below an electrode, and thus, light generated in the light-emitting layer is blocked by the electrode and is not extracted to the outside. Thus, in such a light-emitting device, a light-transmitting electrode is usually used, and light is extracted through the light-transmitting electrode.
In the related art, a known conductive material, such as a layered structure in which oxides of Ni, Co or the like and Au or the like which is contact metal are combined, has been used for the light-transmitting electrode. In recent years, a layered structure, in which a light transmission oxide having high conductivity such as ITO is used and the film thickness of contact metal is significantly thin to increase the transparency, has been used as the light-transmitting electrode, so that light from the light-emitting layer can be efficiently extracted to the outside.
Further, in the light-emitting device in the related art, in order to obtain a high level of light emission luminance, the entire light-emitting layer (semiconductor layer), which is not limited to only the portion directly below the electrode, should uniformly emit light. However, as mentioned above, in the light-emitting device in which the light-transmitting electrode is provided on the semiconductor layer and the bonding pad electrode is provided thereon, electric current is concentrated directly below the bonding pad electrode. Thus, as mentioned above, the light emission effect through the light-emitting layer is concentrated directly below the bonding pad electrode, to thereby lower light emission efficiency, which results in reduction in luminance.
In this regard, in order to prevent electric current from being concentrated directly below the bonding pad electrode in the light-emitting device which is provided with such a light-transmitting electrode, there has been proposed a light-emitting device in which an insulation layer is provided directly below a bonding pad electrode (for example, refer to Patent Documents 1 and 2). In the light-emitting devices as disclosed in Patent Documents 1 and 2, as the insulation layer is provided as mentioned above, electric current diffusion in a transverse direction in a light-transmitting electrode is effectively facilitated, thereby making it possible to enhance light emission efficiency. However, in the light-emitting devices disclosed in Patent Documents 1 and 2, light emission is relatively strong in the proximity of an n-side bonding pad electrode, and there may be a problem in that it is difficult to obtain a superior electric characteristic, and the light emission efficiency cannot necessarily be enhanced.    [Patent Document 1] Japanese Patent No. 3841460    [Patent Document 2] JP-A-2008-192710